>>3171163>The ones with glass are called speed boosters, they give you a full frame picture on a crop sensor.Not true. This is (sometimes) true for adapters that mount old SLR lenses to mirrorless cameras, but I don't think there's any EF-mount adapter that's like this.
>>3171161The adapters with extra optics are necessary for mounts that don't have a compatible flange-focal distance. I.e., the distance between where the lens mounts to the camera body and the actual focal plane.
If a system A has relatively long flange-focal distance compared to system B, you can mount A's lenses on B. Basically, if the difference in flange-focal distance is enough to fit a mount adapter in the middle. Examples of this are the F mount and M42 screw mount.
If the flange focal distance is shorter (or so close that there's still not enough room for the physical size of an adapter), then the adapter means that the lens will be pushed a little further forward than it's expecting, which throws off its focus. If you want your lens to work normally in a situation like that, the adapter needs to have optics in addition to the physical mount to correct for the different flange-focal distance. Sometimes the optics are shitty, and that makes your whole lens system shitty even if you have a nice lens in front of the adapter and a nice sensor behind it, which is why people don't normally do this. Examples of this case would be Minolta MD or Canon FD mounted onto Canon EF.
There ARE adapters for longer-flange-focus cameras that don't have optics, but they won't focus to infinity. Mostly they're only useful if you want to use your lens as a macro lens, because pushing the lens out a little bit lets it focus more closely at the expense of being able to focus further out.
